This invention is concerned with the delivery of cryogenic liquids by way of a cryogenic subcooler system.
Cryogens, such as liquid oxygen (LOX), are stored in well-insulated storage vessels, generally at atmospheric pressure. Although the liquid remains at its normal cryogenic temperature, heat leaking into the storage tanks causes product loss through boil-off. Sometimes, if the tanks are not vented, the heat will be retained by the liquid, but with an increase in pressure. The stored heat will be released when the liquid is vented to atmospheric pressure during transfer of the liquid from the storage vessels. Transfer piping, hoses, valves, and other components of the transfer system, when not in use, also absorb a quantity of heat dependent upon the ambient temperature, as may the vessels that are to receive cryogenic liquid from the storage vessels.
When cryogenic liquid is exposed to these warm components, it is heated immediately and begins to boil, i.e., it rapidly changes from a liquid to a vapor. One volume of LOX, for example, expands to 860 volumes of vapor. This phenomenon is a serious obstacle to the efficient transfer of cryogenic liquids from storage vessels, particularly when the transfer of intermittent and at low flow rates. As a liquid, a cryogen may be transferred with ease, but when it "flashes" to vapor, because of the heat in the transfer lines, etc., the extreme change in volume "chokes" the flow. If the choking vapors can be rapidly removed, however, the transfer system can be cooled to cryogenic temperatures rapidly and thus establish liquid flow without boiling.
Liquid cryogen delivery systems have heretofore been proposed that are intended to deliver cryogenic liquids (essentially free of vapor) to a use point intermittently and at low flow rates. One such system is disclosed in U.S. Pat. No. 4,296,610 to Davis, issued Oct. 27, 1981. This sytem employs a technique known as "subcooling", in which a cooling unit is provided adjacent to the use point for not only cooling cryogen so as to condense the vapor but also to further "subcool" the liquid to a temperature at which the equilibrium vapor pressure is less than the pressure of the liquid. The system of the Davis patent employs a subcooler that is an insulated vessel with a heat exchanger therein. Valves are employed to adjust the flow of cryogen from the heat exchanger during offperiods (when no liquid is to be delivered) to a low value just sufficient to completely absorb the heat added through heat leaks downstream of the subcooler, thereby vaporizing the cryogen so that essentially no liquid cryogen reaches the use point, and to adjust the flow to a higher value during on-periods (when delivery of liquid is desired) so that cryogen is delivered to the use point essentially free of vapor.